The present invention relates to an arrangement for transferring an electrostatic charge within a gravure and flexographic printing unit in order to improve the print quality by polarizing the drops of printing ink on the printing plate cylinder. In the gravure printing unit, the electrostatic charge is applied to the outer circumference of an impression roller, from which it flows away toward the outer circumference of the printing plate cylinder. In the flexographic printing unit, the electrostatic charge is applied to the printing plate cylinder, from which it flows away both toward the substrate transfer roll and toward the back-pressure cylinder. Under the influence of an applied electric field, the ink molecules in the dimples in the printing plate cylinder (gravure printing) or those on the surface of the printing plate cylinder (flexographic printing) are polarized, and the ink droplets experience overall an increase in volume. A flowing electric current is picked up in order to supply the energy needed for the polarization work. As a consequence of the polarization, the ink droplets are attracted by the printing material and, moreover, the transfer of the ink droplets to the printing material led past is promoted by their increase in volume.
Thus, in gravure printing it is ensured to a significantly greater extent that the dimples in the printing plate cylinder are emptied satisfactorily, that is to say the printing ink is applied to the printing material. In flexographic printing, the electrostatic charge has the effect that the printing ink is transferred better from the substrate transfer roll to the printing plate cylinder and on to the printing material. Such arrangements are also referred to as xe2x80x9celectrostatic printing aidsxe2x80x9d; they are used to achieve a full reflection density at all tonal levels and to avoid so-called xe2x80x9cmissing dotsxe2x80x9d. The problem of xe2x80x9cmissing dotsxe2x80x9d occurs in particular with rough printing materials, for example paper webs, having corresponding irregularities.
Electrostatic printing aids of the generic type relevant here have been known for decades (see, for example DE-A-27 09 254; EP-A-0 761 458). FIGS. 1A and 1D, in conjunction with FIG. 1C, show a two-roll system in a gravure printing unit having a multi-layer impression roller 1xe2x80x94but here already having three layers, according to the inventionxe2x80x94the printing plate cylinder 2 and the printing material 4 led between the two over the deflection roll 3. Arranged above the impression roller 1 is a rod-like voltage electrode 5 extending over its entire length. The ink doctor 6 for wiping off excessively applied ink from the printing plate cylinder 2 is indicated. The inking roller and the ink return are situated in an ink trough 7, but are not shown. The voltage electrode 5 is connected to a high-voltage source 8. The circumference of the three-layer impression roller 1 has, on the outside, a semiconductor layer 10 and, underneath the latter, a highly conductive layer 11. Located underneath the highly conductive layer 11, as an electrical insulation from the impression roller core 13, is an insulating layer 12.
FIG. 1B shows a three-roller system which, differing from the above-described two-roller system, has a supporting roll 9, which is preferably electrically insulated, additionally arranged above the multi-layer impression roll 1. Here, the voltage electrode 5 is positioned to the side of the multi-layer impression roller 1.
FIG. 1E, with the electric circuit diagram of the two or three-roller system according to FIGS. 1A to 1D, illustrates the current flow within the electrostatic arrangements. From the high-voltage source 8, a DC voltage U is fed to the voltage electrode 5, and the voltage electrode 5 has the internal resistance R1. The air gap Sxe2x80x94normally of the order of magnitude of about 5 mm to 30 mmxe2x80x94existing between the voltage electrode 5 and impression roller 1, represents the resistance R2. The upper semiconductor layer 10 and the highly conductive layer 11 form the resistances R3, R4. The grounded insulation layer 12 acts as an extra large resistance R5. From the highly conductive layer 11, the current flows through the semiconductor layer 10 which is arranged underneath and which here forms the resistance R6, and onward through the printing material 4, which represents the resistance R7. The grounded printing plate cylinder 2 has virtually the resistance value R8 =0 .
According to Kirchhoff""s law of current distribution, the main proportion of the electric current takes the path of lowest resistance via the highly conductive layer 11, while a small fraction flows directly to the printing material 4 via the semiconductor layer 10. Finally, there is a voltage drop xcex94U between the lower semiconductor layer 10 and the ground E which constitutes the so-called nip voltage, which is critical for the polarization of the ink droplets in the dimples in the printing plate cylinder 2. The current I flows to the ground connection E, starting from the voltage electrode 5.
In order to apply the ink droplets from the dimples as completely and uniformly as possible over the entire width of the printing materialxe2x80x94the web widths can nowadays exceed 3 mxe2x80x94sufficient energy has to be supplied, and the current flow has to be distributed uniformly over the entire impression roller width. In order to satisfy this requirement, the length of the voltage electrode has hitherto depended on the maximum usable width of the printing plate cylinder or of the impression roller, so that a charge distribution which is homogeneous in the impression area is ensured on said printing plate cylinder or impression roller (see DE-A-27 09 254, p. 11, lines 21ff.; OLLECH, Bernd: Tiefdruckxe2x80x94Grundlagen und Verfahrensschritte der modernen Tiefdrucktechnik [gravure printing principles and process steps in modern gravure printing technology], Polygraph Verlag Frankfurt am Main, second edition 1993, p. 343, FIG. 15.49; company publications from Eltex-Elektrostatik GmbH, Weil am Rhein, Germany, xe2x80x9cESA-DIREKTxe2x80x94Eine neue Dimension der elektrostatischen Druckhilfexe2x80x9d [ESA-DIREKTxe2x80x94a new dimension in electrostatic printing aids], publication no.: WP-d/e/f-9043-90/7-20, FIG. 17; and xe2x80x9celtex-Handbuch der Elektrostatischen Disziplinxe2x80x9d [eltex-handbook of the electrostatic discipline], publication no.: xc3x9cp-d-0002-93/12-100, p.32, printing assistance, Figure top right) As a result, use is made of voltage electrodes over 3 m in length. Good print qualities are achieved with such voltage electrodes. However, the drawbacks are the relatively quick contamination of the exposed voltage electrodes, which lead to considerable losses in terms of their effectiveness and ultimately to complete failure, so that the print quality rapidly deteriorates.
In order to obtain the function of the electrostatic arrangements equipped in this way, a contaminated voltage electrode has to be disassembled, cleaned and installed again. This requires a number of personnel, leads to extensive losses in terms of machine downtime and is therefore often delayed in order not to threaten existing delivery times for the printed products.
In order to eliminate the abovementioned disadvantages, the consequence has been that electrostatic printing aids have been developed where, instead of by means of a long, fitted bar electrode, the current was introduced into the rotating shaft of the impression roller (see, for example, DE-A-28 10 452). Then, although the problem of a voluminous voltage electrode had been eliminated, and servicing had therefore been made easier, there nevertheless remains the requirement for frequent cleaning; added to this, however, was an increased outlay in insulating the core of the impression roller with respect to the printing machine.
In the further development, encapsulated electrostatic printing aids were developed, where the current was introduced via the impression roller core and which were protected to the greatest possible extent against contamination, so that freedom from maintenance is virtually provided (see, for example, EP-A-0 115 611, the company publication from Spengler Electronic AG, Biel-Benken, Switzerland: Elektrostatische Druckhilfe [electrostatic printing aids], SR-HELIOFURN 94). These printing aids, which are the most modern to date, entail a relatively high mechanical outlay, which is still acceptable in the case of new printing machines which are equipped with it from the start. In the case of retrofitting older printing machines, which are already in operation, with encapsulated printing aids and introducing the current into the impression roller core, however, the outlay for retrofitting would rise enormously, so that for this purpose the earlier printing aids with long, bar-like voltage electrodes continue to be used (see, for example, most recently the company publication from SHINKO Co., Ltd., Osaka, Japan: ESAPRINT 21, ELECTROSTATIC ASSIST SYSTEM; publication no.: 97043000).
In addition, US-A-3 625 146 discloses electrostatic printing aids in which a roller electrode is fitted to a three-layer impression roller having an external semiconductor layer, a conductor layer located underneath and an insulating layer located underneath the latter and adjacent to the impression roller core. The roller electrode is in direct electrical contact with an exposed annular face of the conductor layer.
In an electrostatic arrangement disclosed by EP-A-0 294 042, a voltage electrode in the form of a brush is in direct electrical contact with a semiconductor layer of a multi-layer impression roller.
DE-U-94 19 540 describes an electrostatic arrangement in which a voltage electrode is arranged at a distance from an outer semiconductor layer of a three-layer impression roller having a highly conductive layer located under the semiconductor layer and an insulation layer located underneath said highly conductive layer and adjacent to the impression roller core. The voltage electrode, which is formed as a sheet-metal part, grating or the like, and the semiconductor layer and the highly conductive layer form a capacitor, which is suitable for transferring alternating voltage. A conventional alternating voltage or an alternating voltage rectified with one diode (=a non-smoothed DC voltage) is applied to the voltage electrode, the alternating voltage component generating the alternating current which can be transferred via the capacitor.
These last electrostatic printing aids are based on a different current transfer principle than the electrostatic arrangement described in DE-A-27 09 254, from which the electrostatic arrangements according to the independent patent claims 1 and 2 are delimited.
In view of the continuing disadvantages of the electrostatic printing aids which exist to date, the invention is based on the object of providing an arrangement where a contaminated voltage electrode can quickly be dismantled, cleaned and installed again by one person. Otherwise, it should be possible to replace the contaminated voltage electrode quickly with a clean electrode, in order to perform the cleaning of the contaminated electrode externally. The outlay on servicing and machine downtimes must be reduced considerably. The arrangement should manage with electrodes of the smallest possible dimension, should in particular be suitable for retrofitting printing machines and the initial procurement costs must be kept low. However, high requirements apply in undiminished form to the print quality.
All of those skilled in the art have previously assumed, as even the latest literature and products show, that if current is introduced via the outer circumference of the impression roller (gravure printing) or the printing plate cylinder (flexographic printing), the use of a voltage electrode extending as far as possible over the entire length of the impression roller or printing plate cylinder is imperative for a homogeneous charge distribution in the imprint region. Surprisingly, it has now been found that if a voltage electrode which is shorter than 50% of the length of the impression roller or the printing plate cylinder is used, and at the same time a three-layer impression roller (gravure printing) or a three-layer printing plate cylinder (flexographic printing) is used, excellent print qualities can be achieved, like those previously achieved only with voltage electrodes of at least virtually complete length. The voltage electrode is fitted at a gap distance from the outer circumference of the impression roller or of the printing plate cylinder and, depending on the high voltage applied and the safety margins associated with this, can be shortened down to about 1% of the previous complete length.
As alternatives to the bar-like voltage electrodes, there were found those which surround the outer circumference of the impression roller or of the printing plate cylinder in a curved shape at a gap distance. The homogeneous distribution of charge over the entire impression area is achieved by utilizing the relatively low-resistance highly conductive layer of the impression roller or of the printing plate cylinder in the axial direction and the opposite high-resistance semiconductor layer in the radial direction.
In order to increase the safety, insulation of the ends of the impression roller or of the printing plate cylinder from their cores is provided by means of the application of an insulating coating, which extends at least from the highly conductive layer into the adjacent regions of the semiconductor layer lying above and the insulating layer lying below. It is also possible for the insulation to be achieved by shortening the highly conductive layer at the ends, and filling the clearance produced by the shortening with the semiconductor or insulating layer.